LEARNED IT · 08/26
Altitude beats every setting, and dispersion is why
A low target is looked at through more air. Beyond blurring, the atmosphere acts as a prism and smears bright objects into a spectrum — a defect no focusing can reach.
"Shoot when it's high" was already on the cheat sheet as a seeing argument: low means more atmosphere means more turbulence means mush. Venus low in the west added the second, sharper reason.
Atmospheric dispersion. Air refracts blue light slightly more than red. Near the horizon you are looking through enough of it that a bright object is smeared into a tiny spectrum — a red fringe along one edge and a blue fringe along the other.
Two things follow:
- It is not chromatic aberration in the telescope, and it is not bad focus. Focusing harder cannot reach it. Mistaking it for either wastes a session.
- The instrument that corrects it is an ADC (atmospheric dispersion corrector), a pair of rotating prisms that cancel the effect. I don't have one.
So on a low target, dispersion sets the ceiling and everything else — focus, collimation, frame count, stacking — is polishing below it. The free fix is wait for altitude, which costs nothing but patience and beats every setting available in the capture software.